1. Technical Field
The present invention relates to a semiconductor device for a high-frequency power amplifier, and more particularly, to a semiconductor device which forms a high-frequency power amplifier accommodated in a transmitter of a mobile communication equipment such as a portable telephone.
2. Related Art
A mobile communication system has been widely utilized for radio-frequency communication between a number of pieces of mobile communication equipment and base stations located at distance in a service area. Such communication system is realized as a portable telephone or the like. A typical portable telephone comprises a transmitter section including a sonic-electric converter, modulator, frequency converter, and radio-frequency (RF) power amplifier.
The high-frequency (RF) power amplifier of a transmitter section of a portable telephone is required to carry out high-quality power-amplification of digitally modulated high-frequency signals of 2 GHz band. Most of conventional high-frequency power amplifiers are formed by single-ended circuits which have a proven track record in hybrid ICs. Recently, various attempts have been made to configure a high-frequency power amplifier by a balanced amplifier circuit such as a differential amplifier circuit comprised of a pair of high-frequency transistors of a common emitter type.
As schematically shown in FIG. 1, a high-frequency power amplifier 200 of transmitter section of a portable telephone is configured to amplify a high-frequency signal Sp, Sn which is balanced-input through bases of a pair of high-frequency transistors 1 and 2, and output the amplified high-frequency signal to inductive load 4 and 5 connected to collectors of these transistors 1 and 2. A transistor 3 connected to emitters of the transistors 1 and 2 serves as a constant current source for determining the bias current for these transistors.
As shown in FIG. 2, each of the transistors 1, 2 and 3 is formed in actual by a transistor cell 12 comprised of parallel-connected high-frequency transistors (hereinafter referred to as transistor elements) 11 each having a desire high-frequency characteristic. More specifically, the transistor elements 11 forming each transistor cell 12 are formed in array on a semiconductor substrate. For instance, each transistor element has a device configuration (not shown) having emitter and collector regions between which a base region is provided. The transistor elements 11 have their emitter electrodes E, base electrodes B and collector electrodes C which are formed as exemplarily shown in FIG. 3 and connected to electrode wiring patterns 13, 14 and 15, respectively. Ordinarily, each transistor cell 12 composed of transistor elements 11 serves as a single transistor having a desired high-frequency characteristic and a required current capacity (rated current).
In the case of a high-frequency power amplifier for use in a portable telephone, each of the transistors 1, 2 and 3 is formed by a group of transistors (hereinafter referred to as a composite transistor) 16 so as to attain a power handling capacity (rated power) corresponding to a maximum transmitting power output, each composite transistor 16 being composed of a required number of parallel-connected transistor cells 12, as exemplarily shown in FIG. 4. A high-frequency signal is power-amplified by the transistors 1 and 2, and the bias current level for the transistors 1, 2 is adjusted by the transistor 3.
The high-frequency power amplifier having the transistors 1 and 2 composed of the composite transistors 16 may produce a low-quality power-amplified high-frequency signal, due to the difference between operation characteristics of the two composite transistors 16. Even if the two groups of the transistor cells forming the two composite transistors 16 are formed monolithically on the semiconductor substrate with the intention of equalizing their operation characteristics, the operation characteristics may vary, in a strict sense, among individual transistor cells 12 due to the differences between positions on the substrate where these transistor cells 12 are formed. In the power amplifier shown in FIG. 4, one group of transistor cells 12 forming one composite transistor 16 serving as the transistor 1 are formed on one end of the semiconductor substrate, whereas another group of transistor cells 12 forming another composite transistor 16 serving as the transistor 2 are formed on another end of the substrate. Due to such a positional difference between these two transistor cell groups, a certain difference appears between their operation characteristics, resulting in low-quality power amplification of a high-frequency signal.
It is preferable for the power-amplified high-frequency signal to satisfy such a requirement on the balance characteristic that "high-frequency signal currents appearing at two output lines of a power amplifier be the same in magnitude and opposite in phase." Depending on a layout of electrode wiring patterns through which associated electrodes of the transistor cells 12 are connected, the power-amplified high-frequency signal can have unsatisfactory balance characteristic.